1. Field of the Invention
The invention relates to gas separation operations. More particularly, it relates to enhanced air and other gas separation operations using preferred zeolitic adsorbents.
2. Description of the Prior Art
For a wide variety of commercial applications in which cryogenic air separation plants may not be economically feasible, pressure swing adsorption (PSA) systems are particularly suitable. For example, PSA systems have been used to supply high purity oxygen for various applications, such as chemical processing, steel mills, paper mills, and lead and gas production operations.
In PSA processing, a feed gas mixture, such as air, containing a more readily adsorbable component and a less readily adsorbable component, e.g., the nitrogen and oxygen components of air, is passed to the feed end of an adsorbent bed capable of selectively adsorbing the more readily adsorbable component at an upper adsorption pressure. The less readily adsorbable component, e.g., oxygen, passes through the bed and is recovered from the discharge end of the bed. Thereafter, the bed is depressurized to a lower desorption pressure for desorption of the more readily adsorbable component, and its removal from the feed end of the bed prior to the introduction of additional quantities of the feed gas mixture for repressurization of the bed and adsorption of the more readily adsorbable component as cyclic adsorption-desorption-repressurization operations are continued in the bed.
Such PSA processing is commonly carried out in multi-bed adsorption systems, with each bed employing the PSA processing sequence on a cyclic basis interrelated to the carrying out of the processing sequence in the other beds of the adsorption system. In PSA systems for the recovery of high purity oxygen product as the less readily adsorbable component of air, each adsorbent bed will commonly contain an adsorbent material capable of selectively adsorbing nitrogen as the more readily adsorbable component, with the selectively adsorbed nitrogen being subsequently desorbed and recovered from the feed end of the bed upon reduction of the pressure of the bed from the upper adsorption pressure to a lower desorption pressure level. PSA systems for the recovery of nitrogen product have likewise been based on the use of adsorbents that selectively adsorb nitrogen from air as the more readily adsorbable component thereof, although other PSA-nitrogen processes are based on the use of oxygen-selective adsorbents, such as various carbon adsorbent materials.
Early PSA air separation systems utilized two or three beds, with well known molecular sieves, e.g., 13X zeolite molecular sieve material, being used as the adsorbent therein. Such zeolitic molecular sieve material, and other such materials, e.g., 5A zeolite molecular sieve material, capable of selectively adsorbing nitrogen from air are equilibrium type adsorbents. In the use of such adsorbents, an adsorption front of the selectively adsorbed nitrogen is formed at the feed end of the bed, and advances toward the discharge, or oxygen product, end of the bed as a result of equilibrium conditions established in the bed of zeolite molecular sieve material between the more readily adsorbable nitrogen and the less readily adsorbable oxygen component of feed air.
While conventional zeolite molecular sieves can be used in PSA operations, specially modified materials can also be employed for improved performance, such as for the improved adsorption of nitrogen from feed air, and the recovery of oxygen or nitrogen as the desired product gas. Thus, the lithium cation forms of conventional zeolite X have been developed for use in PSA processing. Such lithium, i.e., LiX, adsorbent is found to exhibit a highly desirable capacity and selectivity for the adsorption of nitrogen from feed air or other streams containing less polar or less polarizable molecular species, such as oxygen.
LiX adsorbent materials proposed for PSA processing operations are the lithium cation forms of zeolite in which the framework SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio is from about 2.0 to about 3.0, preferably from 2.0 to 2.5, and in which at least about 88%, preferably at least 90%, more preferably at least 95%, of the AlO.sub.2 -tetrahedral units are associated with lithium cations. The nitrogen adsorption properties of such highly exchanged forms of LiX were not predictable from the results obtainable using LiX materials in which 86 equivalent percent or less of the cations are lithium and the remainder are principally sodium ions. Such highly exchanged LiX materials are further described in the Chao patent, U.S. Pat. No. 4,859,217, which recognized that high lithium exchange was required for high nitrogen selectivity and that a 99% LiX (2.0) material had a higher nitrogen capacity than a 99% LiX (2.5) material, although no explanation was provided for this circumstance.
In the Coe patent, U.S. Pat. No. 4,481,018, it is disclosed that mixed cation-exchanged X zeolites and faujasites having a Si/Al ratio of about 1.0 to 1.2 (corresponding to a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of about 2.0 to 2.5) can be used for the separation of nitrogen from gas mixtures. The patent teaches a range of SiO.sub.2 /Al.sub.O.sub.3 ratios and cation compositions for improved gas separations, but does not specify exact SiO.sub.2 Al/.sub.2 O.sub.3 ratios or cation compositions that will result in superior selectivities for the more readily adsorbable component of the feed mixture. Likewise, the patent does not recognize or teach which structural or compositional features will control selectivity in these adsorbent materials.
Sircar et al., U.S. Pat. No. 4,557,736, have described the use of calcium/strontium-exchanged X zeolites as improved adsorbents. The SiO.sub.2 /Al.sub.2 O.sub.3 ratios for enhanced performance are not specified, but ranges are given for calcium, strontium and sodium cation levels. The resulting materials were reported to have higher nitrogen adsorption capacities, lower heats of nitrogen adsorption and improved selectivities relative to non-exchanged precursors.
Lithium exchange was also disclosed in the Coe patent, U.S. Pat. No. 4,925,460, which relates to lithium-exchanged chabazites for air separation. The patent specifies a Si/Al ratio of 2.1-2.8 (corresponding to a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of 4.2-5.6), and a range of lithium exchange levels equal to, or greater than, 65%. Calcium-exchanged chabazites for gas separation are described in the Coe et al. patent, U.S. Pat. No. 4,943,304, which relates to the separation of minor components from bulk gases, and not to air separation or air purification applications. A Si/Al ratio of 1.9-2.3 is disclosed, as well as a special composition of Si/Al ratio =2, cation siting=1, and a cation distribution=1. Both the framework Si/Al ratio and the cations' position and distribution were said to affect the nitrogen adsorption properties of the adsorbent, but the relationship between the Si/Al (SiO.sub.2 /Al.sub.2 O.sub.3) ratio and cation composition to adsorbent sample selectivity, i.e., the composition and/or structure of preferred adsorbent compositions, was not recognized in said patent.
The Coe patent, U.S. Pat. No. 4,544,378, teaches that mixed cation forms of X-type faujasites are advantageous for air separation purposes. Separation factors, determined by a gas chromatography method, are shown to be related to levels of cation exchange and adsorbent sample activation conditions. While higher selectivities are attributed to higher levels of cation exchange in an X (2.5) zeolite, no connection is made to specific compositions or framework structures for enhancing the selective adsorption characteristics of the mixed cation forms of X-type faujasites.
The advantages of mixed cation zeolites for air separation applications have also been recognized in two recently issued patents. Chao, U.S. Pat. No. 5,174,979, teaches the use of lithium/alkaline with metal zeolites of the X and A framework structures. SiO.sub.2 /Al.sub.2 O.sub.3 ratios of about 1.85-3.0 were disclosed for X structures, and ratios of about 1.85-4.0 were disclosed for A structures. For lithium/alkaline earth metal X zeolites, cation ratios of about 95:5-50:50 are disclosed, while cation ratios of about 10:90-70:30 are disclosed for lithium/alkaline earth A zeolites. The Coe patent, U.S. Pat. No. 5,152,813, discloses the use of exchanged X zeolites with a Si/Al ratio of equal or less than 1.5 (SiO.sub.2 /Al.sub.2 O.sub.3 ratio of equal or less than 3.0), having at least binary exchange of lithium and calcium and/or strontium, with preferable ratios of 5-50% calcium and/or strontium ions and 50-90% lithium ions. As with previous disclosures referred to above, these two patents claim ranges of Si/Al (SiO.sub.2 /Al.sub.2 O.sub.3) ratios and cation concentrations, but do not teach specific combinations of framework and cation compositions for the achieving of enhanced performance of zeolites in PSA gas separation operations.
While the art has thus made significant progress in the development of special adsorbents to improve air separation and other PSA gas separation operations, there is a need for further improvement in the adsorbent field. In particular, there is a need to develop PSA air and other gas separation operations utilizing specific preferred zeolite compositions to better satisfy the ever-increasing requirements of a variety of industrial applications for the desirable pressure swing adsorption technology. Such specific preferred zeolite compositions employed in such enhanced PSA gas separation operations will enable enhanced selectivities for the more readily adsorbable component to be achieved, and lower cost zeolite adsorbent compositions to be considered, so as to achieve substantial savings in the operation of practical commercial PSA systems.
It is an object of the invention, therefore, to provide enhanced PSA processing operations and special adsorbents for use therein.
It is another object of the invention to provide enhanced performance in PSA air and other gas separation operations using preferred zeolite adsorbents.
It is a further object of the invention to provide specific combinations of framework and cation compositions capable of superior zeolite performance in PSA gas separation operations.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.